A 1:1 protection scheme is standardized by ITU-T (International Telecommunication Union) (ITU-T Recommendation G8031). In this scheme, a transmission device and a reception device are connected via two lines in order to improve the reliability of communication, and in the initial state, communication is performed using one of the lines, while at the time of maintenance work or failure, communication is performed using the other line. In particular, in the 1:1 protection standardized by the ITU-T, a line can be switched to another line by the unit of VLAN traffic based on a switching request by a user.
In the 1:1 protection standardized by the ITU-T Recommendation G8031, the line switching sections of a transmission device and a reception device are synchronized with each other to switch a line. Meanwhile, another scheme is also proposed, in which a reception device regularly receives frames from the both lines, and 1:1 protection is realized by only switching the line of the transmission device side (see Patent Document 1, for example).
On the other hand, regarding a 1+1 protection scheme rather than the 1:1 protection, Patent Document 2 describes a technique of performing line switching for preventive maintenance based on the status of error correction of a line. Specifically, the transmission side inputs, to a path, a multiframe pattern for detecting a transmission delay difference between the active system and the standby system at the reception side to generate a multiplexed SONET/SDH frame, encapsulates the SONET/SDH frame in a frame for FEC (Forward Error correction), and transmits the result to both the active system line and the standby system line. The reception side includes, in each of the active system and the standby system, an FEC function frame reproduction section which extracts the SONET/SDH frame from the received FEC frame, performs error-correction processing by the FEC function, and then transfers error correction information; a demultiplexing section which terminates the path from the generated SONET/SDH frame and establishes synchronization of the multiframe added at the transmission side; a B3 error detection section which compares, with respect to the frame from the demultiplexing section, a parity operation result for each path with the B3 byte of the next path to detect presence or absence of a B3 error; and a delay memory n which data from the B3 error detection section is written in order to offset a transmission path delay difference between the active system and the standby system and which is read at the timing extracted from multiframe synchronization. Further, the reception side includes a switching control section which switches, between the active system and the standby system, data from a delay memory based on the B3 error and the error correction information.
It should be noted that in this description, a frame of the Ethernet (registered trademark) is simply called as a frame or a packet.
Patent-Document 1: JP 10-145374 A
Patent Document 2: JP 2005-260820 A
In the 1:1 protection standardized by the ITU-T, if the switching target line is a high-speed line having a smaller delay quantity compared with that of the switching source line, there is a problem that a packet loss is caused when switching lines. For example, when a transmission device transmits a packet 1, a packet 2, a packet 3, and a packet 4 in this order to the switching source line, and then transmits subsequent packets, namely, a packet 5, a packet 6, a packet 7, and a packet 8, in this order to the switching target line, if the switching target line is a higher-speed line compared with the switching source line, the packets 5 and 6 reach the reception device before the packets 3 and 4 reach the device, as shown in FIG. 9. As such, if line switching is performed immediately after the packet 2 is received from the switching source line, the packets 3 and 4 will be lost. On the contrary, if line switching is performed immediately after the packet 4 is received from the switching source line, the packets 5 and 6 will be lost.
On the other hand, in the scheme described in Patent-Document 1, as the reception device is able to receive not only the packets 1 to 4 but also the packets 5 to 8, a packet loss can be prevented. However, a state where the packets 5 and 6 are received before the packets 3 and 4 may be caused. As such, there is a problem that the sequence of the packets will be changed.
Further, although switching lines for preventive maintenance based on the status of error correction of the received frames is performed in the 1+1 protection scheme as described in Patent Document 2, a case of applying such a technique to the 1:1 protection scheme has not been found. The grounds are as follows: as the status of error correction of the received frames is only detected by the reception side, it is considered difficult to apply the technique to the 1:1 protection in which path switching is performed at the transmission side; and as a packet loss or a change in the sequence of packets is caused if there is any delay difference between the two lines in the 1:1 protection as described above, preventive maintenance cannot be realized.